Renewable electrical energy that is electrical energy created from alternative sources such as those that are environmentally compatible and perhaps sourced from easily undisruptively available sources such as solar, wind, geothermal or the like is highly desirable. Considering, but not limiting, the example of solar power this is almost obvious. For years, solar power has been touted as one of the most promising for our increasingly industrialized society. Even though the amount of solar power theoretically available far exceeds most, if not all, other energy sources (alternative or not), there remain practical challenges to utilizing this energy. In general, solar power remains subject to a number of limitations that have kept it from fulfilling the promise it holds. In one regard, it has been a challenge to implement in a manner that provides adequate electrical output as compared to its cost. The present invention addresses an important aspect of this in a manner that significantly increases the ability to cost-effectively permit solar power to be electrically harnessed so that an AC output may be a cost-effective source of electrical power whether it be provided for internal use or for public consumption, such as feedback to a grid or the like.
Focusing on solar power as it may be applied in embodiments of the invention, one of the most efficient ways to convert solar power into electrical energy is through the use of solar cells. These devices create a photovoltaic DC current through the photovoltaic effect. Often these solar cells are linked together electrically to make a combination of cells into a solar panel or a PV (photovoltaic) panel. PV panels are often connected in series to provide high voltage at a reasonable current. Voltage, current, and power levels may be provided at an individual domestic level, such as for an individual house or the like. Similarly, large arrays of many, many panels may be combined in a sea of panels to create significant, perhaps megawatt outputs to public benefit perhaps as an alternative to creating a new coal burning power plant, a new nuclear power plant, or the like.
Regardless of the nature of the combination, the output (perhaps of a solar cell or a solar panel, or even combinations thereof) is then converted to make the electrical power most usable since the power converters often employed can use high voltage input more effectively. This converted output is then often inverted to provide an AC output as generally exists in more dispersed power systems whether at an individual domestic or even a public level. In a first stage in some systems, namely, conversion of the alternative source's input to a converted DC, conventional power converters sometimes even have at their input handled by an MPPT (maximum power point tracking) circuit to extract the maximum amount of power from one or more or even a string of series connected panels. One problem that arises with this approach, though, is that often the PV panels act as current sources and when combined in a series string, the lowest power panel can limit the current through every other panel. In a second stage in some systems, namely the inversion function to transform the DC into AC, another problem can be that operation of the conversion at maximum power point (MPP) can be somewhat incompatible with or at least suboptimal for an inverter. Prior to the present invention, it was widely seen that it was just an inherent characteristic that needed to be accepted and that the MPP conversion function was so electrically critical that it was generally accepted as a control requirement that made suboptimization at the inverter level merely a necessary attribute that was perhaps inherent in any converted-inverted system. Perhaps surprisingly, prior to this invention, the goal of optimizing both the MPP conversion function while also optimizing the inversion function was just not seen as an achievable or perhaps at least significant goal. The present invention proves that both such goals can not only be achieved, but the result can be an extraordinarily efficient system.
In understanding (and perhaps defending) the perceived paramount nature of an MPP operation, it may be helpful to understand that, in general, solar cells historically have been made from semiconductors such as silicon pn junctions. These junctions or diodes convert sunlight into electrical power. These diodes can have a characteristically low voltage output, often on the order of 0.6 volts. Such cells may behave like current sources in parallel with a forward diode. The output current from such a cell may be a function of many construction factors and, is often directly proportional to the amount of sunlight. The low voltage of such a solar cell can be difficult to convert to power suitable for supplying power to an electric power grid. Often, many diodes are connected in series on a photovoltaic panel. For example, a possible configuration could have 36 diodes or panels connected in series to make 21.6 volts. With the shunt diode and interconnect losses in practice such panels might only generate 15 volts at their maximum power point (MPP). For some larger systems having many such panels, even 15 volts may be too low to deliver over a wire without substantial losses. In addition, typical systems today may combine many panels in series to provide voltages in the 100's of volts in order to minimize the conduction loss between the PV panels and a power converter. Electrically, however, there can be challenges to finding the right input impedance for a converter to extract the maximum power from such a string of PV panels. Naturally, the input usually influences the output. Input variances can be magnified because, the PV panels usually act as current sources and the panel producing the lowest current can sometimes limit the current through the whole string. In some undesirable situations, weak panels can become back biased by the remainder of the panels. Although reverse diodes can be placed across each panel to limit the power loss in this case and to protect the panel from reverse breakdown, there still can be significant variances in the converted output and thus the inverted input. In solar panel systems, problems can arise due to: non-uniformity between panels, partial shade of individual panels, dirt or accumulated matter blocking sunlight on a panel, damage to a panel, and even non-uniform degradation of panels over time to name at least some aspects. These can all be considered as contributing to the perception that a varying inverted input was at least practically inevitable. Just the fact that a series connection is often desired to get high enough voltage to efficiently transmit power through a local distribution to a load, perhaps such as a grid-tied inverter has further compounded the aspect. In real world applications, there is also frequently a desire or need to use unlike types of panels without regard to the connection configuration desired (series or parallel, etc.). All of this can be viewed as contributing to the expectation of inevitability relative to the fact that the inverter input could not be managed for optimum efficiency.
In previous state-of-the-art system, acceptable efficiency has been at relatively lower levels (at least as compared to the present invention). For example, in the article by G. R. Walker, J. Xue and P. Sernia entitled “PV String Per-Module Maximum Power Point Enabling Converters” those authors may have even suggested that efficiency losses were inevitable. Lower levels of efficiency, such as achieved through their ‘enhanced’ circuitries, were touted as acceptable. Similarly, two of the same authors, G. R. Walker and P. Sernia in the article entitled “Cascaded DC-DC Converter Connection of Photovoltaic Modules” suggested that the needed technologies would always be at an efficiency disadvantage. These references even include an efficiency vs. power graph showing a full power efficiency of approximately 91%. With the high cost of PV panels operation through such a low efficiency converter it is no wonder that solar power has been seen as not yet readily acceptable for the marketplace. The present invention shows that this need not be true, and that much higher levels of efficiency are in fact achievable.
Another less understood problem with large series strings of PV panels may be with highly varying output voltage, the inverter stage driving the grid my need to operate over a very wide range also lowering its efficiency. It may also be a problem if during periods of time when the inverter section is not powering the grid that the input voltage to this stage may increase above regulatory limits. Or conversely, if the voltage during this time is not over a regulatory limit then the final operational voltage may be much lower than the ideal point of efficiency for the inverter. In addition, there may be start-up and protection issues which add significant cost to the overall power conversion process. Other less obvious issues affecting Balance of System (BOS) costs for a solar power installation are also involved. Thus, what at least one aspect of electrical solar power needs is an improvement in efficiency in the conversion stage of the electrical system. The present invention provides this needed improvement.